Luminescent decay and spectra of impurity-activated alkali halides under high pressure

Abstract

The effect of high pressure on the luminescence of alkali halides doped with the transition-metal ions Cu+ and Ag+ and the heavy-metal ions In+ and Tl+ was investigated to 140 kbar. Measurement of spectra allowed the prediction of kinetic properties, and the predictions agree with lifetime data. In view of the localized nature of the electronic transitions, a pressure-dependent single configuration coordinate model is used to interpret the luminescence data. While pressure affects the volume, it also couples to the nontota1ly symmetric coordinates that determine the lifetime in the phosphors studied here. Luminescent kinetics are governed by the assistance of odd phonons for Cu+ and Ag+, and by Jahn-Te11er distortions for In+ and Tl+. Analysis of room-temperature measurements of emission peak location and peak ha1fwidth yields parameters characteristic of the potential wells of transition-metal ions in alkali halides. Using a pressure-dependent model of phonon-assisted transitions, these parameters predict the change in lifetime with pressure. Agreement between calculation and the experimentally determined lifetime is reasonable. The possibilities and limitations of the analysis are discussed. Measurements of steady-state intensity and lifetime were made over a range of pressures (4 to 60 kbar) and temperatures (100 to 3oo0 K) for five aikali halide crystals doped with In+ and n+. The emission spectrum is a doublet (or a triplet in the case of CsI:Tl), caused by a Jahn-Teller splitting of the excited state. The relative intensity distribution of the spectral peaks as a function of temperature and pressure determines the parameters for a model of two levels in "dynamic equilibrium." The same model predicts lifetime changes with temperature and pressure which are in excellent agreement with the data for the In+-doped compounds. For the Tl+-doped compounds, metastable states control the lifetime, and parameters are extracted from the data for a multi-level model. Level splittings, level degeneracies, and intrinsic radiative rates are among the parameters determined in this study. Lifetimes were found from low-light level decay curves recorded after pulsed light excitation. A signal-averaging transient digitizer was used for lifetimes from one microsecond to five seconds. The single-photon counting method was employed for fast lifetimes of one hundred nanoseconds to fifty microseconds.